Batteries are subject to a variety of defects and failure mechanisms that may lead to impaired performance or catastrophic failure, as well as collateral damage to neighboring batteries, electronics, and miscellaneous structures. Exemplary defects and failure mechanisms include internal and external short circuits, thermal runaway, malfunctioning battery casings, and malfunctioning venting systems. These, and other, defects and failure mechanisms may be the result of manufacturing flaws, improper handling or storage, misuse, improper charging (for rechargeable batteries), or the environment in which the batteries operate or are stored. Environmental conditions such as temperature, humidity and pressure all play a significant role in the initiation of a battery failure.
FIG. 1 is a simplified cross-sectional view of a battery 100, for example a lithium ion battery utilizing the 18650 form-factor. Battery 100 includes a cylindrical case 101, an electrode assembly 103, and a cap assembly 105. Case 101 is typically made of a metal, such as nickel-plated steel, that has been selected such that it will not react with the battery materials, e.g., the electrolyte, electrode assembly, etc. For an 18650 cell, case 101 is comprised of a cylinder and an integrated, i.e., seamless, bottom surface 107. Cap assembly 105 includes a battery terminal 109, e.g., the positive terminal, and an insulator 111, insulator 111 preventing terminal 109 from making electrical contact with case 101. Cap assembly 105 typically also includes an internal positive temperature coefficient (PTC) current limiting device and a venting mechanism (neither shown), the venting mechanism designed to rupture at high pressures and provide a pathway for cell contents to escape. Cap assembly 105 may contain other seals and elements depending upon the selected design/configuration. Electrode assembly 103 is comprised of an anode sheet, a cathode sheet and an interposed separator, wound together in a spiral pattern often referred to as a ‘jelly-roll’. An anode electrode tab 113 connects the anode electrode of the wound electrode assembly to the negative terminal while a cathode tab 115 connects the cathode electrode of the wound electrode assembly to the positive terminal. In the illustrated embodiment, the negative terminal is case 101 and the positive terminal is terminal 109. In most configurations, battery 100 also includes a pair of insulators 117/119. Case 101 includes a crimped portion 121 that is designed to help hold the internal elements, e.g., seals, electrode assembly, etc., in place.
In a battery such as that shown in FIG. 1, condensation can easily accumulate in area 123. Condensation accumulation is more likely to occur in applications in which the battery is subjected to a wide range of environments and operating conditions, for example the battery pack in an electric vehicle.
When condensation or water accumulates in area 123, or in a similar region in a battery with a different configuration, electrolytic and galvanic corrosion will typically occur due to the voltage differential and the different materials used for the battery case and the terminal. Electrolytic and galvanic corrosion leads to the rapid corrosion of the terminal, casing, or both. As a result, the life expectancy of the affected battery is greatly reduced. Additionally, as the affected battery corrodes, it may rupture which can lead to the damage or destruction of adjacent cells. Battery corrosion may also cause the affected battery to short circuit, which will not only affect the performance of the battery pack in which the affected battery resides, but may also lead to the battery entering into thermal runaway. Due to the large amount of thermal energy rapidly released during a thermal runaway event, cells in proximity to the affected cell may also enter into thermal runaway, leading to a cascading effect. As a result, power from the battery pack is interrupted and the system employing the battery pack is likely to incur extensive collateral damage due to the scale of thermal runaway and the associated release of thermal energy.
In a conventional battery pack, the battery regions of concern (e.g., region 123) remain susceptible to condensation. For example, FIG. 2 illustrates a portion of a conventional battery pack housing member with a plurality of batteries mounted within the corresponding mounting wells. As shown, the end region of each battery remains open to the environment and, as a result, allows condensation to continue to accumulate near the battery cap assemblies.
One approach to overcoming the electrolytic and galvanic corrosion problem is to apply a potting material to the end region of each battery. While such an approach does prevent corrosion, it prevents access to the battery terminal of the cap assembly (e.g., terminal 109 of FIG. 1). Therefore the potting material must be applied after coupling (e.g., resistance welding or soldering) the battery interconnect to the battery terminal. Unfortunately this approach prevents inspection and/or replacement of a battery interconnect after potting. The potting material may also interfere with the proper functioning of terminal interconnect fuses. Lastly, a large amount of potting material, for example that required to encase a large portion of a battery pack, adds significant weight and cost to the battery pack, thereby making this an undesirable, and in many cases unacceptable, solution.
Accordingly, what is needed is a means for preventing condensation-induced corrosion of a battery, and more specifically for preventing electrolytic and galvanic corrosion from occurring between the battery's cap assembly and casing. The present invention provides such a means.